The present invention relates to a private branch exchange which can connect ordinary telephone sets with dedicated multi-function telephone sets or the like.
The telephone sets which are connected with a private branch exchange as extension end devices comprise many kinds of telephone sets including "standard" telephone sets, and multi-function telephone sets having various functions, etc. One of such multi-function telephone sets is generally called "key telephone". By providing lines for data communication lines (hereinbelow referred to as "data lines") between the exchange and the telephone sets in addition to voice lines, the key telephone serves to communicate several kinds of control data between the exchange and the key telephone set through the data lines. The exchange with which the key telephone can be connected is provided with a voice terminal and a data terminal in one extension port.
Some of such exchanges are constituted so that ordinary telephone sets which do not have data lines can also be connected therewith. Namely, in such an exchange, ordinary telephone sets are connected with the voice lines and the voice circuit section in the exchange has a like circuit arrangement with that of the exchange designed for ordinary telephone sets. Further, the internal voice circuit characteristic viewed from the voice line terminal is also adapted identically or similarly with that of the exchange dedicated to ordinary telephone sets so that the exchange can be used as it is even when the ordinary telephone sets are connected. Therefore, the exchange is adapted so that information of off hook and on-hook is detected as a change in a D.C. loop current like the exchange for ordinary telephone sets. Namely, a predetermined D.C. voltage is applied to the voice line (tip: T-line and ring-line: R-line) by a D.C. power source as in the exchange for ordinary telephone sets.
The above arrangement of the exchange provides the following problem. Conventional exchanges which permit use of both ordinary telephone sets and multi-function telephone must have an operation state changed in accordance with whether an ordinary telephone set is connected or a multi function (key) telephone set is connected. To this end, in connecting a telephone set with the exchange, a switch is provided to set which kind of telephone set is connected with which terminal. Such a setting of the switch is commonly carried out by a service man. Therefore, each time the telephone set is to be changed or the number of the telephone sets is to be increased, the service man must be called. This is very troublesome.